1. Technical Field
The present invention relates generally to a converter, and more particularly to an AC/DC converter and method of correcting power factor.
2. Description of Related Art
Typically, an AC/DC converter is used to convert an alternate current (AC) into a direct current (DC). FIG. 1 shows a conventional AC/DC converter, which includes a rectifier circuit 300 and an output capacitor C. The rectifier circuit 300 converts an alternate current (AC) of an AC power supply into a direct circuit (DC), and the output capacitor C bridges over the outputs of the rectifier circuit 10 and are connected to a loading 400 in parallel. While the AC/DC converter is working, the phases of the input voltage and the input current of the AC are different, which leads to low power factor and poor total harmonic distortion. In addition, the output capacitor C is charged only when the direct current has higher voltage than the output capacitor C, hence the charging time of the output capacitor C is shortened. As a result, the conduction time of diodes in the rectifier circuit 300 is also shortened, which increases the peak value of the conduction current, distorts the waveform of the input current, and lowers the power factor. Low power factor not only wastes energy, but also poses unnecessary burdens for the power supply systems of the power company. Therefore, the AC/DC converter with power factor correction circuit is emerged.
Typically, a conventional power factor correction circuit for AC/DC converters has two types, which are active type and passive type. Active power factor correction circuits control the input current with active switch components, and this type of power factor correction circuits has several advantages, such as the power factor could excess 0.99, and the total harmonic distortion could be less than 10%. And moreover, active power factor correction circuit is compatible with wider range of input voltage, generates stable output voltage, and it's unaffected by the variation of output power. However, active power factor correction circuit has several main drawbacks too, such as higher cost due to additional active switch components required, high electromagnetic noises, and low durability. The industry is eager to overcome these drawbacks. Besides, in order to reduce the ripple of the output voltage of the loading 400, the aforementioned output capacitor C has to adopt a high capacity electrolytic capacitor, but the electrolytic capacitor is easy to leak out the contained electrolyte due to being heated for a long period of time, which shortens life of circuit.